Method and system for optimum channel equalization from a SerDes to an optical module

ABSTRACT

Certain aspects of a method and system for optimum channel equalization between a host Serializer-Deserializer (SerDes) and an optical module may compensate and reduce dispersion loss along an electrical transmit path of a transmitter and an optical transmit path coupled to the transmitter via pre-emphasis. The data degradation as a result of the dispersion loss along the electrical transmit path of the transmitter and the optical transmit path coupled to the transmitter may be recovered by equalizing signals received via an electrical receive path of a receiver communicatively coupled to the transmitter.

CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE

This application is a continuation of application Ser. No. 11/268,246,filed on Nov. 7, 2005, which is a continuation-in-part of applicationSer. No. 10/418,035 filed on Apr. 17, 2003, issued as U.S. Pat. No.7,321,612 on Jan. 22, 2008, which makes reference to, claims priority toand claims benefit from U.S. Provisional Patent Application Ser. No.60/397,599 filed on Jul. 22, 2002.

FIELD OF THE INVENTION

Certain embodiments of the invention relate to channel equalization.More specifically, certain embodiments of the invention relate to amethod and system for optimum channel equalization from aSerializer-Deserializer (SerDes) to an optical module.

BACKGROUND OF THE INVENTION

High speed fiber optic systems may be used in various communicationapplications, for instance in telecommunication over long transmissiondistances. A telecommunication network may be classified into variouslevels such as subscriber networks, regional networks and nationalnetworks. The national networks, for example, may exist betweendifferent cities where there is a greater demand for higher transmissionspeeds, for example, above 5 Gbits/sec. In the national networks, forexample, dispersion may limit the transmission speeds betweentransmitters and receivers. Optical dispersion is a fundamental problemfor high-speed gigabit networks and is of particular importance as bitrates exceed 2.4 Gbits/sec, for example.

Intersymbol interference (ISI) may also be a problem in digitalcommunications in bandwidth-limited links. The ISI over a communicationlink may be the dominant power penalty in the link power budget andeffectively sets the achievable data rate or transmission distance. Themain source of ISI in a fiber-optic system is signal pulse broadeningdue to fiber dispersion. There may be various types of dispersion in afiber-optic system, which may comprise modal dispersion, chromaticdispersion and polarization mode dispersion, for example. In a multimodefiber, different mode groups have different velocities, which is calledmodal dispersion. Chromatic dispersion may be caused by differentwavelengths of light having different velocities. The polarization modedispersion, which may be due to different velocities of differentpolarizations, may be neglected in multimode fibers.

In order to reduce the dispersion effect, several methods have beenproposed and implemented in both electrical and optical domains. Themain criteria for a good dispersion reduction method are small powerpenalty or loss, good integration with current networks, low cost, andadaptability. Dispersion is usually time varying due to environmentalchange such as temperature variation and is related to fiber length.

Further limitations and disadvantages of conventional and traditionalapproaches will become apparent to one of skill in the art, throughcomparison of such systems with some aspects of the present invention asset forth in the remainder of the present application with reference tothe drawings.

BRIEF SUMMARY OF THE INVENTION

A method and system for optimum channel equalization from a SerDes to anoptical module, substantially as shown in and/or described in connectionwith at least one of the figures, as set forth more completely in theclaims.

These and other advantages, aspects and novel features of the presentinvention, as well as details of an illustrated embodiment thereof, willbe more fully understood from the following description and drawings.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram illustrating electrical channel equalizationin an optical communication circuit, in accordance with an embodiment ofthe invention.

FIG. 2A is a block diagram illustrating a small form factor pluggable(SFP) architecture and its compliance points, in accordance with anembodiment of the invention.

FIG. 2B is a block diagram illustrating the pre-emphasis architecture ina serializer-deserializer (SerDes), in accordance with an embodiment ofthe invention.

FIG. 2C is a block diagram illustrating an equalizer architecture, whichmay be utilized in a SerDes, in accordance with an embodiment of theinvention.

FIG. 2D is a block diagram illustrating looping back the electricalreceive path to calculate dispersion loss in an optical communicationcircuit, in accordance with an embodiment of the invention.

FIG. 3A is a graph illustrating return loss with connector for a 40 mmmicrostrip transmission line (MSTL), in accordance with an embodiment ofthe invention.

FIG. 3B is a graph illustrating return loss with connector for a 250 mmMSTL, in accordance with an embodiment of the invention.

FIG. 4A is a graph illustrating loss tangent with connector for a 200 mmstripline (STL), in accordance with an embodiment of the invention.

FIG. 4B is a graph illustrating loss tangent with connector for a 200 mmmicrostrip transmission line (MSTL), in accordance with an embodiment ofthe invention.

FIG. 5A is a graph illustrating an eye diagram for a 25 mm trace lengthat 8.5 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 5B is a graph illustrating an eye diagram for a 25 mm trace lengthat 10.3 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 5C is a graph illustrating an eye diagram for a 200 mm trace lengthat 8.5 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 5D is a graph illustrating an eye diagram for a 300 mm trace lengthat 8.5 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 5E is a graph illustrating an eye diagram for a 200 mm trace lengthat 10.3 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 5F is a graph illustrating an eye diagram for a 300 mm trace lengthat 10.3 GHz over SFP, in accordance with an embodiment of the invention.

FIG. 6 is a flowchart illustrating optimum channel equalization from aSerDes to an optical module, in accordance with an embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Certain aspects of a method and system for optimum channel equalizationbetween a host Serializer-Deserializer (SerDes) and an optical modulemay comprise reducing dispersion loss along an electrical transmit pathof a transmitter and an optical transmit path coupled to the transmittervia pre-emphasis. Signal degradation resulting from the dispersion lossalong the electrical transmit or receive path of the host SerDes and theoptical module may be compensated by equalizing signals received via anelectrical receive path of a receiver communicatively coupled to thetransmitter.

FIG. 1 is a block diagram illustrating electrical channel equalizationin an optical communication circuit, in accordance with an embodiment ofthe invention. Referring to FIG. 1, there is shown a plurality of hostboards 102 a and 102 b, a forward optical path x 118 b and a reverseoptical path x 118 a. The host board 102 a comprises a hostserializer-deserializer (SerDes) 104 a, an optical module 106 a, aforward electrical transmit path w 116 a and a reverse electricalreceive path y 114 a. The host board 202 may be coated with a tracematerial, for example, 8″-12″ FR4 trace material. The host SerDes 104 acomprises a receiver RX 108 a and a transmitter TX 110 a. The opticalmodule 106 a comprises a plurality of optical amplifiers 112 a and 112b. The host board 102 b comprises a host SerDes 104 b, an optical module106 b, a reverse electrical transmit path w 116 b and a forwardelectrical receive path y 114 b. The host SerDes 104 b comprises areceiver RX 108 b and a transmitter TX 110 b. The optical module 106 acomprises a plurality of optical amplifiers 112 c and 112 d.

The forward optical communication link comprises the transmitter TX 110a in the host SerDes 104 a, the forward electrical transmit path w 116a, the optical amplifier 112 b in the optical module 106 a, the forwardoptical path x 118 b, the optical amplifier 112 d in the optical module106 b, the forward electrical receive path y 114 b and the receiver RX108 b in the host SerDes 104 b. The plurality of optical modules 106 aand 106 b may be either fixed or pluggable modules. When the opticalmodule 106 a or 106 b is inserted, a nonlinear element may be introducedin the link. The nonlinear element may not be compensated for by usingonly a receive equalizer. The link may be divided into three segments.For example, the transmit electrical channel may comprise thetransmitter TX 110 a in the host SerDes 104 a and the forward electricaltransmit path w 116 a. The optical channel may comprise the opticalamplifier 112 b in the optical module 106 a, the forward optical path x118 b and the optical amplifier 112 d in the optical module 106 b. Thereceive electrical channel may comprise the forward electrical receivepath y 114 b and the receiver RX 108 b in the host SerDes 104 b. Thetransmitter TX 110 a in the host SerDes 104 a and the transmitter TX 110b in the host SerDes 104 b may be adapted to compensate for dispersionloss by pre-emphasis. The receiver RX 108 a in the host SerDes 104 a andthe receiver RX 108 b in the host SerDes 104 b may be adapted tocompensate for dispersion loss by equalization. The reverse opticalcommunication link comprises the transmitter TX 110 b in the host SerDes104 b, the reverse electrical transmit path w 116 b, the opticalamplifier 112 c in the optical module 106 b, the reverse optical path x118 a, the optical amplifier 112 a in the optical module 106 a, thereverse electrical receive path y 114 a and the receiver RX 108 a in thehost SerDes 104 a.

The transmitter TX 110 a may limit the FR4 transmission loss to, forexample, 7 dB. The applied pre-emphasis may be categorized intodifferent groups based on the channel length and the associateddispersion loss. For example, the applied pre-emphasis may be set to LOWfor a dispersion loss of about 3 dB or less, for example. The appliedpre-emphasis may be set to MEDIUM for a dispersion loss of about 3 dB-5dB, for example. The applied pre-emphasis may be set to HIGH for adispersion loss of about 5 dB-7 dB, for example. The appliedpre-emphasis may be set to OFF if no pre-emphasis is used forcompensation of dispersion loss. The pre-emphasis setting may have aminimum impact on jitter and is adapted to improve transmitter jitterperformance. The receiver RX 108 a may limit the FR4 transmission lossto, for example, about 7 dB.

The host SerDes 104 a and/or 104 b may be adapted to calculate theapplied pre-emphasis by categorizing the dispersion loss along at leastone of the first electrical path, for example, the forward electricaltransmit path w 116 a and the first optical path, for example, theforward optical path x 118 b. The host SerDes 104 a may be adapted tooptimize the applied pre-emphasis by looping back the first electricalpath, for example, the forward electrical transmit path w 116 a throughthe optical module 106 a. The first electrical path, for example, theforward electrical transmit path w 116 a may be looped back to the hostSerDes 104 a via the reverse electrical receive path y 114 a, if thedispersion loss along the first electrical path, for example, theforward electrical transmit path w 116 a is similar to the dispersionloss along the second electrical path, for example, the forwardelectrical receive path y 114 b. The host SerDes 104 a may be adapted tooptimize the applied pre-emphasis based on monitoring a channelinter-symbol interference (ISI) along at least one of the firstelectrical path, for example, the forward electrical transmit path w 116a and the second electrical path, for example, the forward electricalreceive path y 114 b. The host SerDes 104 a may be adapted to optimizethe applied pre-emphasis by receiving the monitored channel ISI.

FIG. 2A is a block diagram illustrating a small form factor pluggable(SFP) architecture and its compliance points, in accordance with anembodiment of the invention. Referring to FIG. 2A, there is shown a hostboard 202. The host board 202 comprises a host SerDes 204, an opticalmodule 206, an electrical transmit path 220 and an electrical receivepath 222. The host SerDes 204 comprises a pre-emphasis block 210 and anequalizer block 208. The optical module 206 may also be known as a smallform factor pluggable (SFP) module or device. The optical module 206comprises a plurality of laser diode (LD) amplifiers LDA 216 and LDA218, an Optical Society of America (OSA) transimpedance amplifier (TIA)OSA-TIA 212 and an OSA laser 214.

The host board 202 may be coated with a trace material, for example,8″-12″ FR4 trace material. The pre-emphasis block 210 may comprisesuitable logic, circuitry and/or code that may be adapted to compensatefor dispersion loss in the electrical transmit path 220. The equalizerblock 208 may comprise suitable logic, circuitry and/or code that may beadapted to compensate for dispersion loss in the electrical receive path222.

The laser diode amplifier LDA 218 may comprise suitable logic and/orcircuitry that may be adapted to amplify or boost the electrical signalreceived from the host SerDes 204 and transmit it to the OSA laser 214.The OSA laser 214 may comprise suitable logic and/or circuitry that maybe adapted to convert the received amplified electrical signal to anoptical signal and transmit the optical signal. The OSA-TIA 212 maycomprise suitable logic, circuitry and/or code that may be adapted toconvert a received optical signal into an electrical signal and transmitthe electrical signal to the LDA 216. The laser diode amplifier LDA 216may comprise suitable logic, circuitry and/or code that may be adaptedto amplify or boost the electrical signal received from the OSA-TIA 212and transmit the amplified electrical signal to the host SerDes 204.

A plurality of compliance points may be defined on the host board 202.The compliance point A 224 may represent the output of the transmitterin the host SerDes 204. The compliance point B′ 226 may represent theinput to the SFP or optical module 206. The compliance point B 228 mayrepresent the input to the laser diode amplifier LDA 218. The compliancepoint C 230 may represent the output from the laser diode amplifier LDA216. The compliance point C′ 232 may represent the output from the SFPor optical module 206. The compliance point D 234 may represent theinput to the receiver in the host SerDes 204.

FIG. 2B is a block diagram illustrating the pre-emphasis architecture ina Serializer-Deserializer (SerDes), in accordance with an embodiment ofthe invention. Referring to FIG. 2B, there is shown a multiplexer block240, a plurality of delay blocks 242 and 244, a plurality of multipliers250 a, 250 b and 250 c, a summer block 246 and a SFP 248.

The plurality of delay blocks 242 and 244 may comprise suitable logic,circuitry and/or code that may be adapted to delay an incoming signal byat least one time period T. The plurality of multipliers 250 a, 250 band 250 c may comprise suitable logic, circuitry and/or code that may beadapted to receive a plurality of signals and generate a multipliedoutput of the received signals to the summer block 246. The summer block246 may comprise suitable logic, circuitry and/or code that may beadapted to sum the received signals from the plurality of multipliers250 a, 250 b and 250 c and generate an output to the SFP 248. The SFP248 may comprise suitable logic, circuitry and/or code that may beadapted to receive an input signal from the summer block 246 on the hostside and generate an output to the line side. The received input on thehost side may be a function of the output of the multiplexer block 240.

The pre-emphasis block 210 (FIG. 2A) may be designed as a feed forwardequalizer (FFE) filter, for example. The output of the summer block 246may be represented by the following relationship:Y(t)=g1*x(t−1)+g2*x(t)+g3*x(t+1)where g1, g2 and g3 are weight coefficients. The weight coefficients g1,g2 and g3 may be adjusted based on an expected printed circuit board(PCB) trace loss.

FIG. 2C is a block diagram illustrating an equalizer architecture, whichmay be utilized in a SerDes, in accordance with an embodiment of theinvention. Referring to FIG. 2C, there is shown a SFP 260, a pluralityof delay blocks 262, 264 and 270, a plurality of multipliers 274 a, 274b, 274 c and 274 d, a summer block 266, a decision block 268 and ademultiplexer 272.

The SFP 260 may comprise suitable logic and/or circuitry that may beadapted to receive an input signal from the optical path on the lineside and generate an electrical signal to the host SerDes 204 on thehost side. The plurality of delay blocks 262, 264 and 270 may comprisesuitable logic, circuitry and/or code that may be adapted to delay anincoming signal by at least one time period T. The plurality ofmultipliers 274 a, 274 b and 274 c may comprise suitable logic,circuitry and/or code that may be adapted to receive a plurality ofsignals and generate a multiplied output of the received signals to thesummer block 266. The summer block 266 may comprise suitable logic,circuitry and/or code that may be adapted to sum the received signalsfrom the plurality of multipliers 274 a, 274 b and 274 c and generate anoutput to the multiplier 274 d. The multiplier 274 d may comprisesuitable logic, circuitry and/or code that may be adapted to receive aplurality of signals from the summer block 266 and the delay block 270and generate an output to the decision block 268. The decision block 268may comprise suitable logic, circuitry and/or code that may be adaptedto receive an input signal from the multiplier 274 d and generate anoutput to the demultiplexer 272. The delay block 270 may in the feedbackpath from the output of the decision block 268 and the input of themultiplier 274 d. The output d1 of the delay block 270 may be an inputto the multiplier 274 d.

The received signal from the optical path with intersymbol interference(ISI) may be processed to retrieve lost data due to dispersion loss andchannel ISI. A decision feedback equalizer (DFE) may be utilized torecover lost data, for example. The DFE may comprise a feed-forwardfilter that may be adapted to filter the received signals. The DFE maycomprise a feedback filter that may be adapted to filter previouslyreceived symbols and cancel their impact on the output of thefeed-forward filter. The feed-forward filtered signals and the feedbackfiltered signals may be combined and input to the decision block 268.The decision block 268 may be adapted to make decisions on a symbol bysymbol basis.

FIG. 2D is a block diagram illustrating looping back the electricalreceive path to calculate dispersion loss in an optical communicationcircuit, in accordance with an embodiment of the invention. Referring toFIG. 2D, there is shown a plurality of host boards 282 a and 282 b, aforward optical path x 298 b and a reverse optical path x 298 a. Thehost board 282 a comprises a host Serializer-Deserializer (SerDes) 284a, an optical module 286 a, a forward electrical transmit path w 296 a,a reverse electrical receive path y 294 a and a reverse electricalreceive path y 294 c. The host board 282 a and/or host board 282 b maybe coated with a trace material, for example, 8″-12″ FR4 trace material.The host SerDes 284 a comprises a receiver RX 288 a and a transmitter TX290 a. The optical module 286 a comprises a plurality of opticalamplifiers 292 a and 292 b. The host board 282 b comprises a host SerDes284 b, an optical module 286 b, a reverse electrical transmit path w 296b and a forward electrical receive path y 294 b. The host SerDes 284 bcomprises a receiver RX 288 b and a transmitter TX 290 b. The opticalmodule 286 a comprises a plurality of optical amplifiers 292 c and 292d.

The blocks in FIG. 2D may be defined similar to the blocks defined inFIG. 1. The host SerDes 284 a may be adapted to loop back the electricaltransmit path w 296 a of the transmitter TX 290 a through the opticalmodule 286 a coupled to the electrical transmit path w 296 a back to thehost SerDes 284 a via the reverse electrical receive path y 294 c, ifthe dispersion loss along the electrical transmit path w 296 a of thetransmitter TX 290 a is similar to the dispersion loss along theelectrical receive path y 294 b of the receiver RX 288 b.

FIG. 3A is a graph 302 illustrating mask return loss with connectorwaveform 304, return loss waveform 306 and SDD1 waveform 308 for a 40 mmmicrostrip transmission line (MSTL), in accordance with an embodiment ofthe invention. The return loss with connector for the waveforms 304, 306and 308 are based on a ball grid array (BGA) package with 2×0.5picofarad (pf) of electrostatic discharge (ESD) diode with a 40 mmmicrostrip transmission line (MSTL), for example. There are around sevenpeaks and troughs in the return loss waveform 306, for example. Thenumber of peaks and troughs for a 40 mm microstrip transmission line(MSTL) are greater than the number of peaks and troughs for a 250 mmmicrostrip transmission line (MSTL). The number of multiple reflectionsfor a 40 mm microstrip transmission line (MSTL) is greater than thenumber of multiple reflections for a 250 mm microstrip transmission line(MSTL) due to line attenuation. The return loss for a 40 mm microstriptransmission line (MSTL) may be around −7 dB, for example.

FIG. 3B is a graph 322 illustrating mask return loss with connectorwaveform 324, return loss waveform 326 and SDD1 waveform 328 for a 250mm microstrip transmission line (MSTL), in accordance with an embodimentof the invention. The return loss with connector for the waveforms 324,326 and 328 are based on a ball grid array (BGA) package with 2×0.5picofarad (pf) of electrostatic discharge (ESD) diode with a 250 mmmicrostrip transmission line (MSTL), for example. The return loss for250 mm microstrip transmission line (MSTL) may be around −11 dB, forexample. The host SerDes 204 (FIG. 2A) may be adapted to insert a lossinto the first electrical path to reduce an effect of multiplereflections between the host SerDes 204 and the high speed opticalmodule 206. The host SerDes 204 may be adapted to utilize passiveattenuation to reduce the effect of multiple reflections between thehost SerDes 204 and the high-speed optical module 206.

FIG. 4A is a graph 402 illustrating loss tangent waveforms withconnector for a 200 mm stripline (STL), in accordance with an embodimentof the invention. The 200 mm stripline may have an impedance Z0=50 ohms,thickness of dielectric B=12.5 mil and a dielectric constant ∈_(r)=4.0,for example. The loss tangent waveform 404 may have a loss tangent Tanδ=0.011 and may be generated for a stripline with width w=5.5 mil,thickness t=0.8 mil, for example. The loss tangent waveform 406 may havea loss tangent Tan δ=0.015 and may be generated for a stripline withwidth w=5 mil, thickness t=0.7 mil, for example. The loss tangentwaveform 408 may have a loss tangent Tan δ=0.019 and may be generatedfor a stripline with width w=4.5 mil, thickness t=0.6 mil, for example.

FIG. 4B is a graph 422 illustrating loss tangent waveforms withconnector for a 200 mm microstrip transmission line (MSTL), inaccordance with an embodiment of the invention. The 200 mm microstriptransmission line (MSTL) may have an impedance Z0=50 ohms, height ofdielectric H=5.8 mil and a dielectric constant ∈_(r)=4.0, for example.The loss tangent waveform 424 may have a loss tangent Tan δ=0.011 andmay be generated for a stripline with width w=10.5 mil, thickness t=0.8mil, for example. The loss tangent waveform 426 may have a loss tangentTan δ=0.017 and may be generated for a stripline with width w=10 mil,thickness t=0.7 mil, for example. The loss tangent waveform 428 may havea loss tangent Tan δ=0.022 and may be generated for a stripline withwidth w=9.5 mil, thickness t=0.6 mil, for example.

Environmental variation may dominate the loss variation as the losstangent may vary from about 0.011-0.022, for example. This variation inloss tangent may be utilized to categorize the applied pre-emphasis. Aminimum loss of 3 dB, for example, may be able to reduce multiplereflections.

FIG. 5A through FIG. 5F are graphs illustrating eye diagrams for aplurality of different trace lengths. In particular, FIG. 5A is a graph502 illustrating an eye diagram 504 for a 25 mm trace length at 8.5 GHzover SFP, in accordance with an embodiment of the invention. The graph502 comprises a marker m3 506 that represents the reciprocal of thefrequency of operation, 8.5 GHz at about 117 ps. A saw tooth waveformmay be input on the X-axis, while the resultant output waveform may begenerated on the Y-axis. FIG. 5B is a graph 512 illustrating an eyediagram 514 for a 25 mm trace length at 10.3 GHz over SFP, in accordancewith an embodiment of the invention. The graph 512 comprises a marker m3516 that represents the reciprocal of the frequency of operation, 10.3GHz at about 97 ps. A saw tooth waveform may be input on the X-axis,while the resultant output waveform may be generated on the Y-axis.There is an increase in channel ISI for a 25 mm trace length at 10.3 GHzover SFP compared to a 25 mm trace length at 8.5 GHz over SFP.

FIG. 5C is a graph 522 illustrating an eye diagram 524 for a 200 mmtrace length at 8.5 GHz over SFP, in accordance with an embodiment ofthe invention. The graph 522 comprises a marker m3 526 that representsthe reciprocal of the frequency of operation, 8.5 GHz at about 117 ps.FIG. 5D is a graph 532 illustrating an eye diagram 534 for a 300 mmtrace length at 8.5 GHz over SFP, in accordance with an embodiment ofthe invention. The graph 532 comprises a marker m3 536 that representsthe reciprocal of the frequency of operation, 8.5 GHz at about 117 ps.The closure of the eye diagram 534 represents increased dispersionlosses for a 300 mm trace length at 8.5 GHz over SFP compared to for a200 mm trace length at 8.5 GHz over SFP.

FIG. 5E is a graph 542 illustrating an eye diagram 544 for a 200 mmtrace length at 10.3 GHz over SFP, in accordance with an embodiment ofthe invention. The graph 542 comprises a marker m3 546 that representsthe reciprocal of the frequency of operation, 10.3 GHz at about 97 ps.FIG. 5F is a graph 552 illustrating an eye diagram 554 for a 300 mmtrace length at 10.3 GHz over SFP, in accordance with an embodiment ofthe invention. The graph 552 comprises a marker m3 556 that representsthe reciprocal of the frequency of operation, 10.3 GHz at about 97 ps.The closure of the eye diagram 554 represents increased dispersionlosses for a 300 mm trace length at 10.3 GHz over SFP compared to for a200 mm trace length at 10.3 GHz over SFP. The increased dispersionlosses reduce the signal to noise ratio (SNR).

Referring to FIG. 5A through FIG. 5F, the respective eye diagrams 504,514, 524, 534, 544 and 554 illustrate that for a trace length over 40mm, the jitter may increase due to multiple reflection. The eye diagrams504, 514, 524, 534, 544 and 554 illustrate signal degradation as tracelength increases. In particular, for trace lengths over 200 mm, there isa noticeable increase in signal degradation. This degradation of thesignal may increase due to channel ISI and may be improved by applyingpre-emphasis.

FIG. 6 is a flowchart illustrating optimum channel equalization from aSerDes to an optical module, in accordance with an embodiment of theinvention. Referring to FIG. 6, exemplary steps may start at step 602.In step 604, the dispersion loss along the electrical and/or opticalpaths may be calculated. In step 606, it may be determined whether anydispersion loss along the electrical and/or optical paths exists. Ifthere is no dispersion loss along the electrical or optical paths,control passes to step 608. In step 608, the pre-emphasis is set to OFFif there is no dispersion loss along the electrical and/or opticalpaths. Control then passes to step 620. If there is dispersion lossalong the electrical and/or optical path, control passes to step 610. Instep 610, it may be determined whether the calculated dispersion lossalong the electrical and/or optical paths is greater than about 3 dB,for example. If the calculated dispersion loss is not greater than about3 dB, control passes to step 612. In step 612, the pre-emphasis is setto LOW. Control then passes to step 620. If the calculated dispersionloss along the electrical and/or optical paths is greater than about 3dB, control passes to step 614. In step 614, it may be determinedwhether the calculated dispersion loss along the electrical and/oroptical paths is greater than about 5 dB, for example. If the calculateddispersion loss along the electrical and/or optical paths is greaterthan about 5 dB, control passes to step 618. In step 618, thepre-emphasis may be set to HIGH. Control then passes to step 620. If thecalculated dispersion loss along the electrical and/or optical paths isnot greater than about 5 dB, control passes to step 616. In step 616,the pre-emphasis may be set to MEDIUM. Control then passes to step 620.

In step 620, the pre-emphasis may be applied along the electrical pathof the transmitter. In step 622, the applied pre-emphasis may beoptimized by looping back the electrical path through an optical moduleback to the host SerDes, if the dispersion loss along the transmitelectrical path is similar to the dispersion loss along the receiveelectrical path. In step 624, the host SerDes may monitor the channelinter-symbol interference (ISI) along at least one of the transmitelectrical path and the receive electrical path. In step 626, the hostSerDes may be adapted to optimize the applied pre-emphasis based on themonitored channel ISI. In step 628, electronic dispersion compensation(EDC) may be utilized to compensate for the dispersion loss along theelectrical path and the optical path. In step 630, equalization may beapplied along the electrical path of a receiver to recover lost data dueto the dispersion loss and channel ISI. In step 632, the host SerDes maybe adapted to insert a loss into the electrical path to reduce an effectof multiple reflections between the host SerDes and an optical module.In step 634, the host SerDes may be adapted to utilize passiveattenuation to reduce the effect of multiple reflections between thehost SerDes and the optical module. In step 636, the host SerDes mayadapted to utilize a printed circuit board (PCB) transmission loss toreduce the effect of multiple reflections between the host SerDes andthe optical module. Control then passes to end step 638.

In accordance with an embodiment of the invention, a system for opticalcommunication may comprise circuitry that reduces dispersion loss alongan electrical transmit path of a transmitter and an optical transmitpath coupled to the transmitter via pre-emphasis. The system maycomprise circuitry that improves signal quality along the electricaltransmit path of the transmitter and the optical transmit path coupledto the transmitter by equalizing signals received via an electricalreceive path of a receiver communicatively coupled to the transmitter.

The system may comprise circuitry that may be adapted to categorize thedispersion loss along the electrical transmit path of the transmitterand the optical transmit path coupled to the transmitter to determine alevel of the pre-emphasis. The system may comprise circuitry that may beadapted to determine the dispersion loss along the electrical transmitpath of the transmitter and dispersion loss along the electrical receivepath of the receiver. The system may comprise circuitry that may beadapted to loop back the electrical transmit path of the transmitterthrough an optical module coupled to the electrical transmit path backto a host Serializer-Deserializer (SerDes), if the dispersion loss alongthe electrical transmit path of the transmitter is similar to thedispersion loss along the electrical receive path of the receiver.

The system may comprise circuitry that may be adapted to determine alevel of the pre-emphasis based on channel inter-symbol interference(ISI) along at least one of: the electrical transmit path of thetransmitter and the electrical receive path of the receiver. The systemmay comprise circuitry that may be adapted to adjust the determinedlevel of pre-emphasis based on the channel ISI. The system may comprisecircuitry that may be adapted to compensate for the dispersion lossalong the electrical transmit path of the transmitter and the opticaltransmit path coupled to the transmitter utilizing electronic dispersioncompensation.

The system may comprise circuitry that may be adapted to insert a lossinto the electrical transmit path of the transmitter to mitigate effectsof multiple reflections between a host Serializer-Deserializer (SerDes)and an optical module. The system may comprise circuitry that may beadapted to mitigate the effects of the multiple reflections between thehost Serializer-Deserializer (SerDes) and the optical module utilizing aprinted circuit board (PCB) transmission path loss. The system maycomprise circuitry that may be adapted to mitigate the effects of themultiple reflections between the host Serializer-Deserializer (SerDes)and the optical module utilizing passive attenuation.

Accordingly, the present invention may be realized in hardware,software, or a combination of hardware and software. The presentinvention may be realized in a centralized fashion in at least onecomputer system, or in a distributed fashion where different elementsare spread across several interconnected computer systems. Any kind ofcomputer system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computer system with a computerprogram that, when being loaded and executed, controls the computersystem such that it carries out the methods described herein.

The present invention may also be embedded in a computer programproduct, which comprises all the features enabling the implementation ofthe methods described herein, and which when loaded in a computer systemis able to carry out these methods. Computer program in the presentcontext means any expression, in any language, code or notation, of aset of instructions intended to cause a system having an informationprocessing capability to perform a particular function either directlyor after either or both of the following: a) conversion to anotherlanguage, code or notation; b) reproduction in a different materialform.

While the present invention has been described with reference to certainembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted withoutdeparting from the scope of the present invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the present invention without departing from its scope.Therefore, it is intended that the present invention not be limited tothe particular embodiment disclosed, but that the present invention willinclude all embodiments falling within the scope of the appended claims.

What is claimed is:
 1. A method for optical communication, the methodcomprising: routing a signal to be transmitted along a forwardelectrical path of a transmitter, wherein the forward electrical pathdirectly routes an electrical signal from the transmitter to an opticalmodule for coupling to an optical path; adjusting a level ofpre-emphasis applied for the forward electrical path based on calculateddispersion loss along a loop electrical path, wherein the loopelectrical path routes an electrical signal from the transmitter to theoptical module over the forward electrical path and loops back to thetransmitter over an electrical receive path from the optical module; andapplying the adjusted pre-emphasis, by a transmit feed-forward filter,to a new signal to be transmitted along the forward electrical path ofthe transmitter.
 2. The method of claim 1, further comprising adjustingthe level of pre-emphasis applied for the forward electrical path basedon intersymbol interference monitored on the forward electrical path. 3.The method of claim 1, wherein the calculated dispersion loss issubstantially similar to a dispersion loss along an electrical receivepath of a receiver, wherein the electrical receive path of the receiverdirectly couples the receiver to an optical module of the receiver. 4.The method of claim 1, further comprising setting coefficients of thetransmit feed-forward filter based on the calculated dispersion lossalong the loop electrical path.
 5. The method of claim 1, furthercomprising applying equalization to a received signal using a decisionfeedback equalizer.
 6. The method of claim 1, further comprisinginserting a loss into the forward electrical path to reduce an effect ofmultiple reflections between the transmitter and the optical module. 7.The method of claim 1, further comprising utilizing passive attenuationto reduce an effect of multiple reflections between the transmitter andthe optical module.
 8. A system for optical communication, the systemcomprising: one or more circuits that are operable to route a signal tobe transmitted along a forward electrical path of a transmitter, whereinthe forward electrical path directly routes an electrical signal fromthe transmitter to an optical module for coupling to an optical path;said one or more circuits further operable to adjust a level ofpre-emphasis applied for the forward electrical path based on calculateddispersion loss along a loop electrical path, wherein the loopelectrical path routes an electrical signal from the transmitter to theoptical module over the forward electrical path and loops back to thetransmitter over an electrical receive path from the optical module; andsaid one or more circuits further operable to apply the adjustedpre-emphasis, by a transmit feed-forward filter, to a new signal to betransmitted along the forward electrical path of the transmitter.
 9. Thesystem of claim 8, wherein said one or more circuits are furtheroperable to adjust the level of pre-emphasis applied for the forwardelectrical path based on intersymbol interference monitored on theforward electrical path.
 10. The system of claim 8, wherein thecalculated dispersion loss is substantially similar to a dispersion lossalong an electrical receive path of a receiver, wherein the electricalreceive path of the receiver directly couples the receiver to an opticalmodule of the receiver.
 11. The system of claim 8, wherein said one ormore circuits are further operable to set coefficients of a transmitfeed-forward filter based on the calculated dispersion loss along theloop electrical path.
 12. The system of claim 8, wherein said one ormore circuits are further operable to apply equalization to a receivedsignal using a decision feedback equalizer.
 13. The system of claim 8,wherein said one or more circuits are further operable to insert a lossinto the forward electrical path to reduce an effect of multiplereflections between the transmitter and the optical module.
 14. Thesystem of claim 8, wherein said one or more circuits are furtheroperable to utilize passive attenuation to reduce an effect of multiplereflections between the transmitter and the optical module.
 15. Anon-transitory computer readable medium having a computer program withexecutable instructions, when executed by a processor, cause theprocessor to: direct a signal to be transmitted along a forwardelectrical path of a transmitter, wherein the forward electrical pathdirectly routes an electrical signal from the transmitter to an opticalmodule for coupling to an optical path; adjust a level of pre-emphasisapplied for the forward electrical path based on calculated dispersionloss along a loop electrical path, wherein the loop electrical pathroutes an electrical signal from the transmitter to the optical moduleover the forward electrical path and loops back to the transmitter overan electrical receive path from the optical module; and apply theadjusted pre-emphasis to a new signal to be transmitted along theforward electrical path of the transmitter.
 16. The non-transitorycomputer readable medium of claim 15, further causing the processor toadjust the level of pre-emphasis applied for the forward electrical pathbased on intersymbol interference monitored on the forward electricalpath.
 17. The non-transitory computer readable medium of claim 15,wherein the calculated dispersion loss is substantially similar to adispersion loss along an electrical receive path of a receiver, whereinthe electrical receive path of the receiver directly couples thereceiver to an optical module of the receiver.
 18. The non-transitorycomputer readable medium of claim 15, further causing the processor toset coefficients of a transmit feed-forward filter based on thecalculated dispersion loss along the loop electrical path.
 19. Thenon-transitory computer readable medium of claim 15, further causing theprocessor to insert a loss into the forward electrical path to reduce aneffect of multiple reflections between the transmitter and the opticalmodule.
 20. The non-transitory computer readable medium of claim 15,further causing the processor to utilize passive attenuation to reducean effect of multiple reflections between the transmitter and theoptical module.